This invention relates to a tracking device for a helical scanning VTR in which recording tracks are formed at an angle to the longitudinal direction of a magnetic tape or a magnetic recording and reproducing system in which concentric or spiral recording tracks are formed on a magnetic disc and, more particularly, to a tracking device for displacing a magnetic head in the direction of the track width according to tracking information.
In the usual tracking device of this type, a magnetic head is mounted on the tip of a bimorph element (which is comprised of a piezoelectric element), and the center position of the magnetic head gap is held coincident with the center position of the track at all times by applying a predetermined voltage to the bimorph element according to tracking information.
In this tracking device, however, the mechanical strength of the bimorph element itself is comparatively low. Therefore, the magnetic head is liable to be greatly deviated from its regular position by an external force such as a force produced when the tip of the magnetic head is brought into contact with the recording surface of a magnetic tape or a magnetic disc. In such a case, it is necessary for restoring the magnetic head to the optimum operating position to apply a considerably large correcting voltage to the bimorph element. This means that the dynamic range of an amplifier for supplying the correcting voltage must be considerably increased, which is disadvantageous from the standpoint of cost. The deviation mentioned above also gives an off-set to a tracking servo system. In order to absorb this off-set, it is necessary to extremely increase the servo gain of the tracking servo system. To do so, there are problems in view of ensuring the stability of the servo system. The bimorph element is formed by cementing together two thin ceramics, or it comprises a piezoelectric element and a non-piezoelectric film such as a metal film or a polymer film cemented to the non-piezoelectric element. In order to cause a great displacement of the magnetic head, therefore, the bimorph element must have a considerable length. To increase the length of the bimorph element, the length of the ceramics must also be increased. For this reason, the ceramics must also be quite thick in order to ensure its mechanical strength. In this case, a considerably high drive voltage must be applied to the bimorph element in order to obtain sufficient operation.
Because of the relationship of the thickness, length, drive voltage and deformation of the bimorph element as mentioned above, it is difficult to design a bimorph element which is well-suited for tracking.